Turbocharging an engine allows the engine to provide power similar to that of a larger displacement engine. Thus, turbocharging can extend the operating region of an engine. Turbochargers function by compressing intake air in a compressor via a turbine operated by exhaust gas flow. Under certain conditions, the flow rate and pressure ratio across the compressor can fluctuate to levels that may result in noise disturbances, and in more severe cases, performance issues and compressor degradation. Such compressor surge may be mitigated by one or more compressor bypass valves (CBVs). The CBVs may recirculate compressed air from the compressor outlet to the compressor inlet, and thus may be arranged in a passage which is coupled to the intake upstream of the compressor and downstream of the compressor in some examples. In some examples, continuous CBVs (CCBVs) may be used, which provide a continuous and continually variable circulation flow from downstream of the compressor to upstream of the compressor. CCBVs may provide boost control and compressor surge avoidance, and may further prevent objectionable audible noise. However, incorporation of such valves can add significant component and operating costs to engine systems.
Engines may also include one or more aspirators may be coupled in an engine system to harness engine airflow for generation of vacuum, for use by various vacuum consumption devices that are actuated using vacuum (e.g., a brake booster). Aspirators (which may alternatively be referred to as ejectors, venturi pumps, jet pumps, and eductors) are passive devices which provide low-cost vacuum generation when utilized in engine systems. An amount of vacuum generated at an aspirator can be controlled by controlling the motive air flow rate through the aspirator. For example, when incorporated in an engine intake system, aspirators may generate vacuum using energy that would otherwise be lost to throttling, and the generated vacuum may be used in vacuum-powered devices such as brake boosters. While aspirators may generate vacuum at a lower cost and with improved efficiency as compared to electrically-driven or engine-driven vacuum pumps, their use in engine intake systems has traditionally been constrained by intake manifold pressure. Whereas conventional vacuum pumps produce a pumping curve which is independent of intake manifold pressure, pumping curves for aspirators arranged in engine intake systems may be unable to consistently provide a desired performance over a range of intake manifold pressures. Further, if an aspirator is large enough to replace a conventional vacuum pump, it may flow too much air into the intake manifold for economical fuel use. Some approaches for addressing this issue involve arranging a valve in series with an aspirator, or incorporating a valve into the structure of an aspirator. An opening amount of valve is then controlled to control the motive air flow rate through the aspirator, and thereby control an amount of vacuum generated at the aspirator. By controlling the opening amount of the valve, the amount of air flowing through the aspirator and the air flow rate can be varied, thereby adjusting vacuum generation as engine operating conditions such as intake manifold pressure change. However, again, adding valves to engine systems can add significant component and operating costs.
The inventors herein have identified parallel, valved aspirator arrangements which, when incorporated in an engine system, may advantageously be controlled to provide selectable, discrete levels of vacuum generation during non-boost conditions as well as discrete levels of continuous compressor bypass flow during boost conditions. In one example embodiment, the aspirator arrangement bypasses an intake compressor (e.g., the aspirator arrangement is coupled to the intake passage both upstream and downstream of the compressor), and includes exactly two aspirators having different throat flow areas. An aspirator shut-off valve arranged in series with each aspirator of the aspirator arrangement may be controlled to allow or disallow flow through the corresponding aspirator, such that multiple discrete flow levels through the aspirator arrangement may be achieved (or, in the case of continuously variable aspirator shut-off valves, even more flow levels may be achieved). For example, when intake manifold pressure is below a threshold (e.g., non-boost operation), a combined motive flow rate through the aspirator arrangement may be controlled based on engine vacuum needs and intake manifold pressure. During such conditions, it may be desirable to divert at least some intake airflow around the compressor and through the aspirator arrangement, for example if engine vacuum replenishment is needed. In some examples, the aspirators in the aspirator arrangement may be positioned such that maximum vacuum generation is achieved during bypass flow through the aspirator arrangement from upstream of the compressor to downstream of the compressor. In contrast, when intake manifold pressure is above a threshold (e.g., boost operation), vacuum generation may be less urgent that reduction of compressor surge. Because the pressure differential during boost enables recirculation flow through the aspirator arrangement (e.g., flow from downstream of the compressor to upstream of the compressor), the combined motive flow rate through the aspirator from downstream of the compressor to upstream of the compressor may be controlled based on compressor surge, e.g. such that an increasing combined motive flow rate is provided with increasing compressor surge. Advantageously, even during reverse flow through an aspirator (e.g., flow from a mixed flow outlet of the aspirator to the motive inlet of the aspirator, in the case of an aspirator with an asymmetrical flow geometry designed to maximize flow in one direction), some vacuum may be generated due to the venturi effect. Accordingly, the technical result achieved by the aspirator arrangement described herein includes simultaneous compressor surge reduction and vacuum generation during certain engine operating conditions.
Many additional advantages may be achieved by the embodiments described herein. For example, because multiple, parallel aspirators are used, each aspirator may have a relatively small flow diameter and yet the arrangement can still achieve an overall motive flow rate commensurate with that of a single larger aspirator when needed. The relatively small flow diameters of the aspirators enable the use of smaller, cheaper valves controlling their motive flow. Further, relative flow diameters of the parallel aspirators may be strategically selected such that the valves of the aspirators may be controlled to achieve a desired set of discrete levels of motive flow through the arrangement. Furthermore, because the combined motive flow rate through the aspirator arrangement is controllable via the valves, conditions where the motive flow through the aspirators may cause air flow greater than desired may be reduced. Thus, since air flow rate greater than desired can lead to extra fuel being injected, fuel economy may be improved by use of the aspirator arrangement.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.